1. Field of Invention:
This invention relates to a sheet feeder for a printer, which is driven by a motor used also for a printing head moving mechanisms, and has a mechanism for varying the feeding amount of the paper sheet.
2. Description of the Related Art:
The printer plays an important role as one of the peripheral devices in computer and office automation systems.
The printer automatically feeds the paper sheet (called "sheet" hereinafter) when operated by the motor, and sometimes feeds the sheet in response to the user's manual request. Specifically, when printing is completed on one line, the sheet is automatically fed to another line by the operation of the motor. On the other hand, the sheet is fed by manually turning a sheet feed roller so as to load a sheet or to adjust a print start position.
Usually, a sheet feeding motor is independent from a motor for operating other mechanisms in the printer. A stepping motor or a servo-motor is widely used for this purpose, so that the sheet feeder can easily vary the feeding amount of the sheet as desired.
During normal operation, the printer alternately makes a printed record of information on one line and feeds the sheet to a predetermined extent. The printer also performs sheet feeding, but only when there is a blankline or when the printing is completed.
A sheet feeding mechanism normally begins feeding the sheet immediately after the printing is completed on one line. Specifically, the sheet is fed for one line space each time a printing head reciprocates once (this is called "printing cycle" hereinafter). On the other hand, when the printing head does not operate, such as for blanklines, the printer can feed the sheet to a desired extent contrary to the one-line-feed per printing cycle rule. It is possible to shorten the operating time of the printer if the sheet feeder is designed to feed three blank lines per printing cycle, for example.
With this sheet feeding mechanism, it is very important to vary the feeding amount of the sheet in response to the operation of the printer so as to accelerate its operation.
To vary the sheet feeding amount in a simple manner as described above, the stepping motor or servo-motor is widely used so as to drive the sheet feeding mechanism independently of other mechanisms in the printer. Such a motor enables the printer to speedily feed the sheet for blanklines as desired while no printing is carried out.
The printer is also required to be compact in size and light in weight as well as assuring a short operation time. There is a great demand for much smaller and lighter computer and office automation systems and their associated peripheral devices. Such a demand is likely to become intense as portable computers and word processors become more popular in the future.
The provision of an independent sheet feeding motor is, however, contradictory to the foregoing demand for smaller and lighter devices.
A variety of proposals have been made, in which a mechanism is sometimes used to operate a sheet feed motor and a printing head perpendicularly to the sheet feeding direction, or a ribbon feed motor is also operated for the sheet feeding. Thereby, the number of motors to be used can be decreased, which makes the printer compact in size, light in weight and less expensive.
However, since one motor is commonly used for a plurality of mechanisms, it becomes impossible to operate the motor so as to feed the sheet only when necessary. For example, it is not necessary to rotate the motor during the operation of the printing head since no sheet is fed. However, the motor should be rotated to operate the printing head which performs printing while it is being moved. Therefore, some measures should be taken when one motor is used for a plurality of mechanisms according to their functions. Generally, a sheet feed roller shaft is rotated via a power transmission mechanism including a clutch mechanism. To feed the sheet, the sheet feed roller shaft is driven by the motor via the clutch mechanism which is connected to the power transmission mechanism only when it is necessary to feed the sheet.
The sheet feeding mechanism is also required to rotate the sheet feed roller in response to the user's manual loading of a sheet into the printer. In such a case, it is impossible to rotate the sheet feed roller shaft manually when the cam mechanism is incorporated in the power transmission mechanism as described above. It is therefore very difficult for the user to load the sheet into the printer.
When one motor is commonly used for many devices as described above, it is however impossible to vary the rotation amount of the motor so as to feed the sheet to a desired extent as is done by an independent sheet feeding motor.
To meet the demand for a light and compact device, a printing head is sometimes so reduced in size that one line of letters is printed in two reciprocations of the printing head. Specifically, the upper half of the letters is printed by the printing head during a first reciprocation, and the lower half is printed during a second reciprocation thereof. In such a case, the sheet feeding mechanism has to feed the sheet not only for each normal line and blanklines but also for half-line for the lower half line printing.